I is not possible to change your physical flesh into another being. Magic doesn't trancend the laws of physics, it works with them in harmony on a scale that most of humanity doesnt acknowledge or understand.

Learn to be content with your species, and if you feel something isn't right in your life change it for the better. Grow spiritually, do charity work, anything along positive lines if you feel ashamed of your blood. Wanting to descend into the sea woln't bring you any Real peace. That has to be worked for.

no, you can believe all you want, but you cannot use magic to change physics, time and/or space.if you could, i wouldn't be a human, nor would i be living in this century. you could do it, say, your astral form, but in this reality i'm afraid it can't be done. still, fun to try, and it wouldn't hurt... hopefully.